Generally, in production of group III nitride based semiconductor crystal substrates, a group III nitride based semiconductor layer is formed on a sapphire substrate. However, due to difference in lattice constant between a group III nitride based semiconductor and sapphire, a large number of dislocations are unavoidably generated in the produced group III nitride based semiconductor crystal substrates. Among such dislocations, screw dislocations result in generation of vacancies having a diameter of a few to several nanometers, which are called micropipes and which penetrate through the layer in the thickness direction. If such a group III nitride based semiconductor is used to produce a semiconductor device, micropipes may cause current leakage, to thereby deteriorate performance of the semiconductor device. Therefore, reduction of dislocation density is preferred.
Possible approaches for reducing dislocation density are known as methods for producing group III nitride based semiconductors disclosed in Patent Documents 1 and 2. In these methods, a group III nitride based semiconductor layer is etched to form etch pits, and another group III nitride based semiconductor layer is epitaxially grown on the etched group III nitride based semiconductor layer so that the etch pits are filled with the epitaxial layer. According to these methods, dislocation density of the upper group III nitride based semiconductor layer can be lowered as compared with that of the lower group III nitride based semiconductor layer.
Etching techniques for forming etch pits employed in the production methods are also disclosed. Patent Document 1 discloses an etching process employing ammonia or hydrogen gas, whereas Patent Document 2 discloses a wet etching process employing phosphoric acid, sulfuric acid, or KOH, and an etching process employing HCl gas, Cl2 gas, or BCl3 gas.
Patent Document 1 does not disclose the shape of etch pits. Patent Document 2 discloses that the shape of the formed etch pits is conical, hexagonal pyramid, bowl-like, partial-spherical, or a complex shape thereof.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2002-367909
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2002-261027